While I had absolutely no problem installing QT 6 on my iMac, I noticed that the prefs panel was unchanged and in French (I am French). As I had seen that the QT 6 preview was "English only", I looked around to find out wat was the problem.

The problem seems to be that the QT 6 installation doesn't remove the old components before upgrading, so the old international resources are left after the upgrade! If your primary setting is not English, you end up using them with the new prefs panel (which is different from that of QT5, so this is not good!). I had to remove them manually, but I guess the best answer is to remove the QT 5 prefs pane (in /System -> Library -> PreferencePanes) before installing QT 6.

[Editor's note: I haven't installed QT6 on any system, English or non-English, so I can't specifically vouch for this hint. It seemed worth posting as a general heads-up, though, for those who are considering a QT6 Public Preview installation.]

The best with installation that does require english system is to turn the system on english. It will avoid all the mess doing manually the removal of files you don't know about. I already got this problem and that is how I moved around it. With Mac OS X it is so easy to go from a language from another...
Go to Pref system/international and there make the change to english and just log out. When you come back to your account, it's in english and you can do your installation. Change again the prefs in the international pref pane and log out to come back in french or whatever... That is it !
Maybe cooler manner to do a likely-english installation.
Yours,

I'm not certain it is the solution to this particular case: the problem here is that the OLD language resources are kept. With an application, it is easy to remove localization with the "get info" panel, but with a preference pane, it is impossible, so you have to do it manually... ;-)